Field of the Invention
The present invention relates to a color image forming apparatus, such as a laser printer, a photocopier, or a facsimile, which employs an electrophotography recording method.
Description of the Related Art
In general, in color image forming apparatuses, a phenomenon which is so-called “white gap” in which an irregular white gap, which is not intended to be generated, is generated between adjacent images of different colors has occurred. This phenomenon occurs in the following situation. Specifically, an electrostatic latent image obtained by a rapidly changing potential of a surface of a photoconductor drum, that is an image edge portion, is generated on the photoconductor drum. Then, when this portion is developed by a developing apparatus, a developed image having a width smaller than that of a developed image intended to be formed is generated. In an image including a cyan band and a black band which are adjacent to each other, for example, although the cyan band and the black band should be closely adjacent to each other, a gap is generated between the cyan band and the black band in a final image generated on a recording material since a developed image of the cyan band and a developed image of the black band are formed with smaller widths.
FIG. 12 is a diagram used to explain the white gap in detail and shows a state of an electric field generated between a developer roller and a photoconductor drum. A smaller width of a developed image in an image developing portion causes a white gap since the electric field winds around an edge portion of an electrostatic latent image formed in an electrostatic portion on a photoconductor drum.
To address this problem, a method for performing minute emission using a light emitting element of a laser scanner on a non-image section (non-toner-image-forming unit) in an entire printable region of the photoconductor drum to the extent that toner attachment does not occur has been used, so that the width of the image is prevented from being small. Hereinafter, this method is referred to as “background exposure”, “non-image-section minute emission”, or the like.
Note that an object for performing the non-image-section minute emission is not limited to the prevention of generation of the white gap. For example, as disclosed in Japanese Patent Laid-Open No. 2003-312050, the non-image-section minute emission is performed for making contrast of a transfer potential smaller and preventing image disturbance which occurs in a gap between the developing roller and the photoconductor drum in accordance with aerial discharge. Specifically, the non-image-section minute emission is not performed for a limited usage.
Here, as a concrete method for performing the non-image-section minute emission, a method for changing a duty ratio of a pulse wave which is referred to as a PWM (Pulse Width Modulation) method has been proposed in Japanese Patent Laid-Open No. 2003-312050. In this method, a light emitting element of a laser scanner emits light in a non-image section with a pulse width corresponding to an intensity of minute emission in synchronization with an image clock which has a fixed frequency.
In recent years, there is a demand for higher-quality images generated by color image forming apparatuses. Therefore, in addition to control of an intensity of emission light corresponding to an image section, appropriate control of an intensity of light of minute emission in the non-image section is required.